Re: [PATCH] c++: generic targs and identity substitution [PR105956]

[Patrick Palka <ppalka@redhat.com>]

On Fri, 1 Jul 2022, Jason Merrill wrote:

> On 6/29/22 13:42, Patrick Palka wrote:
> > In r13-1045-gcb7fd1ea85feea I assumed that substitution into generic
> > DECL_TI_ARGS corresponds to an identity mapping of the given arguments,
> > and hence its safe to always elide such substitution.  But this PR
> > demonstrates that such a substitution isn't always the identity mapping,
> > in particular when there's an ARGUMENT_PACK_SELECT argument, which gets
> > handled specially during substitution:
> > 
> >    * when substituting an APS into a template parameter, we strip the
> >      APS to its underlying argument;
> >    * and when substituting an APS into a pack expansion, we strip the
> >      APS to its underlying argument pack.
> 
> Ah, right.  For instance, in variadic96.C we have
> 
>     10	template < typename... T >
>     11	struct derived
>     12	  : public base< T, derived< T... > >...
> 
> so when substituting into the base-specifier, we're approaching it from the
> outside in, so when we get to the inner T... we need some way to find the T
> pack again.  It might be possible to remove the need for APS by substituting
> inner pack expansions before outer ones, which could improve worst-case
> complexity, but I don't know how relevant that is in real code; I imagine most
> inner pack expansions are as simple as this one.

Aha, that makes sense.

> 
> > In this testcase, when expanding the pack expansion pattern (idx + Ns)...
> > with Ns={0,1}, we specialize idx twice, first with Ns=APS<0,{0,1}> and
> > then Ns=APS<1,{0,1}>.  The DECL_TI_ARGS of idx are the generic template
> > arguments of the enclosing class template impl, so before r13-1045,
> > we'd substitute into its DECL_TI_ARGS which gave Ns={0,1} as desired.
> > But after r13-1045, we elide this substitution and end up attempting to
> > hash the original Ns argument, an APS, which ICEs.
> > 
> > So this patch partially reverts this part of r13-1045.  I considered
> > using preserve_args in this case instead, but that'd break the
> > static_assert in the testcase because preserve_args always strips APS to
> > its underlying argument, but here we want to strip it to its underlying
> > argument pack, so we'd incorrectly end up forming the specializations
> > impl<0>::idx and impl<1>::idx instead of impl<0,1>::idx.
> > 
> > Although we can't elide the substitution into DECL_TI_ARGS in light of
> > ARGUMENT_PACK_SELECT, it should still be safe to elide template argument
> > coercion in the case of a non-template decl, which this patch preserves.
> > 
> > It's unfortunate that we need to remove this optimization just because
> > it doesn't hold for one special tree code.  So this patch implements a
> > heuristic in tsubst_template_args to avoid allocating a new TREE_VEC if
> > the substituted elements are identical to those of a level from ARGS.
> > It turns out that about 30% of all calls to tsubst_template_args benefit
> > from this optimization, and it reduces memory usage by about 1.5% for
> > e.g. stdc++.h (relative to r13-1045).  (This is the maybe_reuse stuff,
> > the rest of the changes to tsubst_template_args are just drive-by
> > cleanups.)
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?  Patch generated with -w to ignore noisy whitespace changes.
> > 
> > 	PR c++/105956
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.cc (tsubst_template_args): Move variable declarations
> > 	closer to their first use.  Replace 'orig_t' with 'r'.  Rename
> > 	'need_new' to 'const_subst_p'.  Heuristically detect if the
> > 	substituted elements are identical to that of a level from
> > 	'args' and avoid allocating a new TREE_VEC if so.
> > 	(tsubst_decl) <case TYPE_DECL, case VAR_DECL>: Revert
> > 	r13-1045-gcb7fd1ea85feea change for avoiding substitution into
> > 	DECL_TI_ARGS, but still avoid coercion in this case.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp0x/variadic183.C: New test.
> > ---
> >   gcc/cp/pt.cc                             | 113 ++++++++++++++---------
> >   gcc/testsuite/g++.dg/cpp0x/variadic183.C |  14 +++
> >   2 files changed, 85 insertions(+), 42 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic183.C
> > 
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 8672da123f4..7898834faa6 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -27,6 +27,7 @@ along with GCC; see the file COPYING3.  If not see
> >        Fixed by: C++20 modules.  */
> >     #include "config.h"
> > +#define INCLUDE_ALGORITHM // for std::equal
> >   #include "system.h"
> >   #include "coretypes.h"
> >   #include "cp-tree.h"
> > @@ -13544,17 +13545,22 @@ tsubst_argument_pack (tree orig_arg, tree args,
> > tsubst_flags_t complain,
> >   tree
> >   tsubst_template_args (tree t, tree args, tsubst_flags_t complain, tree
> > in_decl)
> >   {
> > -  tree orig_t = t;
> > -  int len, need_new = 0, i, expanded_len_adjust = 0, out;
> > -  tree *elts;
> > -
> >     if (t == error_mark_node)
> >       return error_mark_node;
> >   -  len = TREE_VEC_LENGTH (t);
> > -  elts = XALLOCAVEC (tree, len);
> > +  const int len = TREE_VEC_LENGTH (t);
> > +  tree *elts = XALLOCAVEC (tree, len);
> > +  int expanded_len_adjust = 0;
> >   -  for (i = 0; i < len; i++)
> > +  /* True iff no element of T was changed by the substitution.  */
> > +  bool const_subst_p = true;
> > +
> > +  /* If MAYBE_REUSE is non-NULL, as an optimization we'll try to reuse and
> > +     return this TREE_VEC instead of allocating a new one if possible.
> > This
> > +     will either be ARGS or a level from ARGS.  */
> > +  tree maybe_reuse = NULL_TREE;
> > +
> > +  for (int i = 0; i < len; i++)
> >       {
> >         tree orig_arg = TREE_VEC_ELT (t, i);
> >         tree new_arg;
> > @@ -13580,56 +13586,90 @@ tsubst_template_args (tree t, tree args,
> > tsubst_flags_t complain, tree in_decl)
> >         else if (ARGUMENT_PACK_P (orig_arg))
> >   	new_arg = tsubst_argument_pack (orig_arg, args, complain, in_decl);
> >         else
> > +	{
> >   	  new_arg = tsubst_template_arg (orig_arg, args, complain, in_decl);
> >   +	  /* If T heuristically appears to be a set of generic template
> > +	     arguments, set MAYBE_REUSE to the corresponding level from
> > +	     ARGS.  */
> > +	  if (maybe_reuse == NULL_TREE && orig_arg != NULL_TREE)
> > +	    {
> > +	      if (TEMPLATE_PARM_P (orig_arg))
> > +		{
> 
> This doesn't handle the case of variadic template parameters, which are
> represented in the generic args with a pack expansion.  If this is a choice,
> there should be a comment about it.

AFAICT substituting such a pack expansion will never be an identity
transform -- the relevant targ during tsubst_pack_expansion will be an
ARGUMENT_PACK, and we'll return the TREE_VEC from that ARGUMENT_PACK
instead of the ARGUMENT_PACK itself, so there's no way we can reuse (a
level from) 'args' in this case.  So variadic template parameters act as
an optimization barrier here unfortunately.  I can add a comment to that
effect, and perhaps explicitly give up in this case by also setting
maybe_reuse to error_mark_node in the PACK_EXPANSION_P branch of
tsubst_template_args?

> 
> > +		  int level, index;\f
> > +		  template_parm_level_and_index (orig_arg, &level, &index);
> > +		  if (index == i && TMPL_ARGS_DEPTH (args) >= level)
> > +		    maybe_reuse = TMPL_ARGS_LEVEL (args, level);
> > +		  else
> > +		    maybe_reuse = error_mark_node;
> > +		}
> > +	      else
> > +		/* T is not a set of generic template arguments; use
> > +		   error_mark_node to denote this.  */
> > +		maybe_reuse = error_mark_node;
> > +	    }
> > +	}
> > +
> >         if (new_arg == error_mark_node)
> >   	return error_mark_node;
> >           elts[i] = new_arg;
> >         if (new_arg != orig_arg)
> > -	need_new = 1;
> > +	const_subst_p = false;
> >       }
> >   -  if (!need_new)
> > +  if (const_subst_p)
> >       return t;
> >   +  /* If ARGS and T are both multi-level, then substituted T may be
> > +     identical to ARGS (if each level was pairwise identical).  */
> > +  if (maybe_reuse == NULL_TREE
> > +      && TMPL_ARGS_HAVE_MULTIPLE_LEVELS (t)
> > +      && TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args))
> > +    maybe_reuse = args;
> > +
> > +  /* Return MAYBE_REUSE and avoid allocating a new TREE_VEC if the
> > substituted
> > +     result is identical to it.  */
> > +  if (NON_ERROR (maybe_reuse) != NULL_TREE
> > +      && TREE_VEC_LENGTH (maybe_reuse) == len
> > +      && std::equal (elts, elts+len, TREE_VEC_BEGIN (maybe_reuse)))
> > +    return maybe_reuse;
> > +
> >     /* Make space for the expanded arguments coming from template
> >        argument packs.  */
> > -  t = make_tree_vec (len + expanded_len_adjust);
> > -  /* ORIG_T can contain TREE_VECs. That happens if ORIG_T contains the
> > +  tree r = make_tree_vec (len + expanded_len_adjust);
> > +  /* T can contain TREE_VECs. That happens if T contains the
> >        arguments for a member template.
> > -     In that case each TREE_VEC in ORIG_T represents a level of template
> > -     arguments, and ORIG_T won't carry any non defaulted argument count.
> > +     In that case each TREE_VEC in T represents a level of template
> > +     arguments, and T won't carry any non defaulted argument count.
> >        It will rather be the nested TREE_VECs that will carry one.
> > -     In other words, ORIG_T carries a non defaulted argument count only
> > +     In other words, T carries a non defaulted argument count only
> >        if it doesn't contain any nested TREE_VEC.  */
> > -  if (NON_DEFAULT_TEMPLATE_ARGS_COUNT (orig_t))
> > +  if (NON_DEFAULT_TEMPLATE_ARGS_COUNT (t))
> >       {
> > -      int count = GET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (orig_t);
> > +      int count = GET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (t);
> >         count += expanded_len_adjust;
> > -      SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (t, count);
> > +      SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (r, count);
> >       }
> > -  for (i = 0, out = 0; i < len; i++)
> > +  for (int i = 0, out = 0; i < len; i++)
> >       {
> > -      tree orig_arg = TREE_VEC_ELT (orig_t, i);
> > +      tree orig_arg = TREE_VEC_ELT (t, i);
> >         if (orig_arg
> >   	  && (PACK_EXPANSION_P (orig_arg) || ARGUMENT_PACK_P (orig_arg))
> >             && TREE_CODE (elts[i]) == TREE_VEC)
> >           {
> > -          int idx;
> > -
> >             /* Now expand the template argument pack "in place".  */
> > -          for (idx = 0; idx < TREE_VEC_LENGTH (elts[i]); idx++, out++)
> > -            TREE_VEC_ELT (t, out) = TREE_VEC_ELT (elts[i], idx);
> > +	  for (int idx = 0; idx < TREE_VEC_LENGTH (elts[i]); idx++, out++)
> > +	    TREE_VEC_ELT (r, out) = TREE_VEC_ELT (elts[i], idx);
> >           }
> >         else
> >           {
> > -          TREE_VEC_ELT (t, out) = elts[i];
> > +	  TREE_VEC_ELT (r, out) = elts[i];
> >             out++;
> >           }
> >       }
> >   -  return t;
> > +  return r;
> >   }
> >     /* Substitute ARGS into one level PARMS of template parameters.  */
> > @@ -14965,32 +15005,21 @@ tsubst_decl (tree t, tree args, tsubst_flags_t
> > complain)
> >     	    if (!spec)
> >   	      {
> > -		int args_depth = TMPL_ARGS_DEPTH (args);
> > -		int parms_depth = TMPL_ARGS_DEPTH (DECL_TI_ARGS (t));
> >   		tmpl = DECL_TI_TEMPLATE (t);
> >   		gen_tmpl = most_general_template (tmpl);
> > -		if (args_depth == parms_depth
> > -		    && !PRIMARY_TEMPLATE_P (gen_tmpl))
> > -		  /* The DECL_TI_ARGS in this case are the generic template
> > -		     arguments for the enclosing class template, so we can
> > -		     shortcut substitution (which would just be the identity
> > -		     mapping).  */
> > -		  argvec = args;
> > -		else
> > -		  {
> >   		argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl);
> > -		    /* Coerce the innermost arguments again if necessary.  If
> > -		       there's fewer levels of args than of parms, then the
> > -		       substitution could not have changed the innermost args
> > -		       (modulo level lowering).  */
> > -		    if (args_depth >= parms_depth && argvec !=
> > error_mark_node)
> > +		if (argvec != error_mark_node
> > +		    && PRIMARY_TEMPLATE_P (gen_tmpl)
> > +		    && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec))
> > +		    /* We're fully specializing a template declaration, so
> > +		       we need to coerce the innermost arguments corresponding
> > +		       to the template.  */
> >   		    argvec = (coerce_innermost_template_parms
> >   			      (DECL_TEMPLATE_PARMS (gen_tmpl),
> >   			       argvec, t, complain,
> >   			       /*all*/true, /*defarg*/true));
> >   		if (argvec == error_mark_node)
> >   		  RETURN (error_mark_node);
> > -		  }
> >   		hash = spec_hasher::hash (gen_tmpl, argvec);
> >   		spec = retrieve_specialization (gen_tmpl, argvec, hash);
> >   	      }
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic183.C
> > b/gcc/testsuite/g++.dg/cpp0x/variadic183.C
> > new file mode 100644
> > index 00000000000..3938e52e0e7
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/variadic183.C
> > @@ -0,0 +1,14 @@
> > +// PR c++/105956
> > +// { dg-do compile { target c++11 } }
> > +
> > +template<int... Ns> struct list;
> > +
> > +template<int... Ns> struct impl {
> > +  static const int idx = 0;
> > +  using type = list<(idx + Ns)...>;
> > +
> > +  static constexpr const int* a[2] = {(Ns, &idx)...};
> > +  static_assert(a[0] == &idx && a[1] == &idx, "");
> > +};
> > +
> > +template struct impl<0, 1>;
> 
>